Don't enable /GX by default

The /GX flag is disabled unless explicitly specified on the command
line.  This partially addresses PR26698.

llvm-svn: 261537
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index fd647a4..b04ef6c 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -5813,8 +5813,10 @@
     }
   }
   // The /GX, /GX- flags are only processed if there are not /EH flags.
+  // The default is that /GX is not specified.
   if (EHArgs.empty() &&
-      Args.hasFlag(options::OPT__SLASH_GX, options::OPT__SLASH_GX_)) {
+      Args.hasFlag(options::OPT__SLASH_GX, options::OPT__SLASH_GX_,
+                   /*default=*/false)) {
     EH.Synch = true;
     EH.NoUnwindC = true;
   }